The use of high purity 2,2-bis(4-hydroxyphenyl) propane, herein referred to as bisphenol-A, as a reactant in the preparation of subsequent formulations such as the preparations of polycarbonate resins is well known in the art. One method for obtaining the purity needed of the bisphenol-A is to distill crude bisphenol-A. In such distillations, a residue is obtained, comprising various by-products and these primarily include higher condensation products of bisphenol-A, condensation products of phenol and acetone produced in the original bisphenol-A formation, colored substances, isomers of bisphenol-A, and the like. Unfortunately, for yield loss purposes, the residues contain 20-60% of bisphenol-A, itself. Prahl et al, U.S. Pat. No. 3,290,390, disclose the addition of phenol to such residues and simultaneously contacting this resulting mixture with an acidic agent such as hydrogen chloride at between room temperature and 150.degree. C. to produce thereform bisphenol-A. However, the method of Prahl et al requires, according to their examples, 16 hours or more reaction time.
It has now been discovered that it is possible to recover the bisphenol-A inherently contained, and in a vastly shorter period of time simply by treating the residue with phenol in the absence of an acidic reagent, such as hydrogen chloride, sulfuric acid, phosphoric acid, and the like. The process of this invention yields bisphenol-A as a solution for subsequent recovery such as by cooling to produce crystals of a 1:1 molar phenol adduct, from which the bisphenol-A can then be recovered, by procedures known per se.
In practice, commercial plants utilize distillation of bisphenol-A as a step in the purification process. The bottoms from the distillation are called "tars" and conventionally these are disposed of by burning. This represents a serious loss in yield because tars contain from 20-60% bisphenol-A plus the isomerizable by-products. Experiments have shown that 28-71% of the bisphenol-A in a typical tar can be recovered by this process. The mother liquor from the crystallization can be stripped of phenol (for recycle) in a separate column, and the diminished quantity of "tar" remaining can be burned or otherwise utilized.